Embodiments of the present embodiments relate to a programmable electrostatic discharge (ESD) protection circuit. Preferred embodiments of the circuit is intended for use at input, output, input-output, or power supply terminals of an integrated circuit.
Referring to FIG. 1A, there is an ESD protection circuit of the prior art as disclosed by Hwang in U.S. Pub. No. 2102/0092798. The circuit of FIG. 1A illustrates a zenner diode 100 connected in series with a PNP bipolar transistor 104. Resistor 102 is connected as a shunt resistor at the base of PNP transistor 104 to inhibit conduction during normal circuit operation. FIG. 1B discloses the effect of the series connection is to shift the current-voltage characteristic from curve 106 without zenner diode 100 to curve 108 with zenner diode 100. Hwang specifically discloses that the ESD protection circuit has a breakdown voltage equivalent to the bipolar transistor's breakdown voltage plus the zenner diode's breakdown voltage. (paragraph [0021]).
One of the problems of the circuit of FIG. 1A is that it shifts the entire curve 106 to curve 108, and, therefore, may produce a breakdown voltage that exceeds the damage threshold of other circuit components during an ESD event. Various embodiments of the present invention are directed to solving this problem and improving operation of the ESD protection circuit without increasing process complexity.